Priority encoders provide signals indicative of the leading one ("1") or zero ("0") in a binary word. Usually, a priority encoder will have as many output lines as the length or number of bits in a binary word, and the voltages on the output lines indicate the leading one or zero in the binary word. For example, a priority encoder to indicate the leading one in an 8-bit word may have eight output lines in which at most one output line has a HIGH voltage (the others are at a LOW voltage), where the one output line with a HIGH voltage indicates the position of the leading one.
Within a priority encoder circuit, there may be one or more groups (or stacks) of serially connected transistors which bring various nodes to a LOW (ground) voltage, depending upon the bit values of the word. To discharge (or pull down) a node to LOW, a transistor stack provides a conducting path between the node and ground. An output circuit coupled to these nodes may then provide the necessary signals indicating the leading zero or one in the word.
In a domino (or dynamic) type priority encoder circuit, the nodes are charged (or pulled up) to a HIGH voltage during a pre-charge state or phase of a clock signal, and as then some or all of the nodes are discharged by various transistor stacks during an evaluation state or phase of the clock signal. The speed at which a domino priority encoder can operate may be limited by the speed at which the various nodes can be discharged. For a given technology, this speed may be increased (i.e., discharge time is decreased) if the stack depths of the transistors (i.e., the number of serially connected transistors in a conducting path between a node and ground) are reduced. For a static type priority encoder circuit, it also may be desirable to reduce the stack depths to increase speed. Also, for CMOS (Complementary Metal Oxide Semiconductor) technology, reducing stack depths may be desirable due to the so-called body effect.
Therefore, it is desirable to reduce the transistor stack depths in priority encoder circuits.